1. Field of the Invention
The present invention relates to dispersion compensation using an electrical-dispersion compensating circuit.
2. Description of the Related Art
Recently, demand has increased for the introduction of next-generation 40-Gb/s optical transmission systems having an equivalent transmission distance and frequency utilization efficiency as 10-Gb/s systems. Therefore, research and development of a differential phase shift keying (DPSK) scheme that is excellent in its optical signal noise ratio (OSNR) resistance and non-linearity resistance is actively being conducted to realize the above.
In addition to the above modulating scheme, research and development of a differential quadrature phase shift keying (DQPSK) scheme having a feature of narrow spectrum (high frequency utilization efficiency) is also actively being conducted. The DQPSK scheme is a scheme of simultaneously transmitting two digital signals that are phase-shift keyed, using a signal light beam having one frequency.
The DQPSK scheme needs only a half of the generally used pulse repetition frequency (for example, 20 GHz) against a data rate for transmission (for example, 40 Gb/s). Therefore, compared to a conventional NRZ keying scheme, etc., the signal spectrum width of the DQPSK scheme is halved and this scheme is excellent in terms of its frequency utilization efficiency, frequency dispersion resistance, device transmission properties, etc. Therefore, in the field of optical transmission systems, application of the DQPSK scheme is commonly considered especially for high-speed transmission systems having data rates exceeding 40 Gb/s.
A typical optical receiver realizing the DQPSK scheme, such as that disclosed in Japanese Patent Application Laid-Open Publication Nos. 2004-516743 and 2007-20138, includes a pair of Mach-Zehnder interferometers that correspond to an “A” branch and a “B” branch. Two output terminals of each Mach-Zehnder interferometer are connected to a balanced photo diode (PD) to recover transmitted data.
On the other hand, concerning ultra-high-speed optical transmission systems, the realization of a system is hindered by degradation of the wave form caused by wavelength dispersion and wave polarization mode dispersion in optical components such as optical fibers and optical amplifiers. Therefore, a technique is used to compensate the degradation of the wave form caused by the wavelength dispersion and the wave polarization mode dispersion. Conventionally, dispersion compensation is executed using mainly optical components such as that disclosed in U.S. Pat. No. 6,807,321.
Meanwhile, recently, an electrical-dispersion compensation (EDC) device using an electric circuit is drawing attention from the perspective of the need to reduce apparatus size and cost concerning optical transmission systems (see, e.g., the Japanese Patent Application Laid-Open Publication Nos. 2004-516743 and 2007-20138, and U.S. Pat. No. 6,807,321 above). The electrical-dispersion compensation device is configured by a transversal filter having plural taps. The level of compensation by the device is varied by adjusting tap coefficients, thereby compensating the degradation of the wave form caused by the wavelength dispersion and wave polarization mode dispersion.
To adjust the tap coefficients, for example, algorithms are used that incrementally correct the tap coefficients such that an equalized error signal becomes minimal using training data, which is known data and user data, which is unknown data, such as a minimum square error (MSE) method, a zero forcing (ZF) method, or a modified zero forcing (MZF) method (see, e.g., Japanese Patent Application Laid-Open Publication No. 2001-14804).
However, with the above conventional techniques and those covered by, for example, Mc Ghan, D., et al. in “Electrical Dispersion Compensation,” OWK1, OFC 2006; Nakamura, M., et al. in “Electrical PMD equalizer ICs for a 40-Gbit/s transmission,” TuG4, OFC 2004; and Franz, B., et al. in “43 Gbit/s SiGe Based Electronic Equalizer for PMD and Chromatic Dispersion Mitigation,” We1.3.1, ECOC 2005, a problem arises in that differences in delay time occur among electrical signals input into an identifying circuit due to the initial variations among elements provided between an optical branching unit and the identifying circuit of an optical receiving apparatus and variation over time thereof. When differences in delay time occur among the electrical signals, the margin for phase synchronization among the electrical signals becomes small with respect to the identifying circuit and the identification of each optical signal becomes difficult. Therefore, a problem arises in that the receiving property is degraded. The phase margin decreases as the speed of an optical signal increases and, therefore, the degradation of the receiving property due to the differences in delay time among the electrical signals becomes prominent.
When differences in delay time among the electrical signals become large, the sign order of the electrical signals become inversed in the identifying circuit and optical signal identification becomes impossible. The occurrence of differences in delay time among the electrical signals may be suppressed by elaborating the skew specification of each of the elements provided between the branching unit and the identifying circuit for an optical signal. However, in addition to an increased cost of the apparatus, a problem arises in that differences in delay time among the electrical signals due to the variation over time of the elements can not be coped with.
Differences in delay time among the electrical signals may be compensated by providing a variable phase shifter that supplies a variable delay amount to the electrical signals on one path of each of the electrical signals. However, when the variable phase shifter is provided, a problem arises in that the apparatus becomes large and the cost of the apparatus increases. A variable phase shifter usually imparts, to an electrical signal, a delay that corresponds to a given electric current value or a given voltage value. However, a problem arises in that it is difficult to precisely compensate the differences in delay time among the electrical signals by universal control of the current value or the voltage value.